I see this sort of phrase thrown around quite a lot and find it a little depressing. Ideas are certainly not worthless, unless your only measure of "worth" is producing a product that can be sold for money.

Still, ideas have to be examinated, developed, tested, debated and adjusted. "Plans are useless, but planning is essencial". Thinking about ideas, discussing and researching similar concepts is very important, but the idea itself is so small in comparison to what it can accomplish and the amount of work it requires to have a minimal impact, it's mostly useless.